Cameras/Video
The largest consideration next to connectivity is probably camera “Stuff.” Media cards, batteries, card readers and cables are good things to have spares of; pretty horrible to be halfway through a day’s activities and find that you have no more room for pictures, or have the camera battery run out of juice. It is just as bad to finish off the first day of a trip and realize that you have no way to transfer your shots to a drive and no more media cards! (More on that next.) A spare camera battery (or rechargeables) and charger are handy indeed.
Storage
Photography, and even more, videography can use a lot of storage space; multiple media cards are good: better a few small ones or one really big one. A large media card may never need to be removed. Having 3 or 4 smaller ones decreases the chance of a failure or loss meaning you lose everything.
Copy or move your pictures/movies to your laptop daily, or if you have a netbook with a small drive, get a portable USB drive. Small 2.5” external drives are pretty cheap, may be more durable than the physically larger 3.5” drives and are a lot smaller and lighter. A 320gb drive is around $100, which is probably money well spent for any traveler who takes a lot of pictures. Make sure to get one that is USB powered; why deal with yet another power cord!
If you don’t want to deal with transferring files, get a bunch of media cards and some decent cases. Enough media cards, and you can just swap out cards and go through everything when you get home.
If you are thinking about buying some of the devices, I personally suggest choosing cameras and devices that all support the same format card if possible; the convenience of having a dSLR, camcorder and laptop that ALL support the same media card is huge.
DVD Players
Standalone dvd players have become less common as laptops have become a staple of the modern “techy”, but they are still around. Don’t forget the cords and/or batteries for these too. If you DO use a laptop for watching dvds (or let the kids use one) consider copying the dvd’s you want to the laptop’s drive if you have the room; hard drives use a LOT less battery power than dvd drives! Virtual drive imaging software lets you load up a “fake” dvd drive with an image of the disk stored on the hard drive if needed.
MP3 Players
Most MP3 players can charge through a usb cord, and the same cord will often fit cameras and portable hard drives. Bring what you need and make SURE you have it. If you think you will be transferring music while away, bring whatever is needed for that.
Electronics Bag
A single, separate bag for your electronic devices may make it easier to keep control of all the cords, plugs, cards and batteries you need when traveling. Considering the cost of these devices, it may also minimize the risk of theft or loss although it DOES mean that if a theft occurs you are likely to lose everything. I suggest getting one that is padded and sturdy, and then keeping it with you at ALL times, of course!